1. Field of the Invention
The present invention relates to a method of manufacturing a multi-channel optical module, and more particularly, to a method of manufacturing a parallel optical transmitter module and a parallel optical receiver module which respectively transmits and receives optical signals using multi-channel optical arrayed elements, in the field of optical communications.
2. Description of the Related Arts
With the rapid spread of the Internet in recent years, the transmission capacity available for each service provider in the backbone optical network has drastically increased. As a result, the demand for optical modules for intra-devices or inter-devices connection with large capacity, low cost, and low power consumption, has been increasing in offices where large-capacity routers and multi optical transmitters are used.
The optical modules which satisfy the above requirements are often proposed in recent years, and especially from the view point of low price and low power consumption, there have been well known the types of the optical modules using surface light-emitting elements such as VCSEL (Vertical Cavity Surface Emitting Laser) as a source of luminescence, and surface photo acceptance elements such as a PIN photodiode for a photoelectric converter receiving light. Furthermore, multi-channel optical modules are often used, which have plural channels of optical elements arrayed with a pitch of 250 μm for mass storage, utilizing the characteristics of VCSEL's low power consumption. Recent multi-channel optical modules require a transmission rate of 10 Gbit/s or more per channel because of increasing demand for mass storage. Also, with the demand for high optical coupling, a micro-lens array is used for this type of optical module to enable high optical coupling with arrayed optical elements and optical fiber array. In order to prevent stray light among neighboring channels, the maximum lens diameter of micro lens array is 250 μm which is equal to the channel pitch between the optical elements. There are the following requirements for manufacturing optical modules, since the lens diameter, the optical element, and the channel pitch of lens array are as small as 250 μm:
a) For optical coupling of a light-emitting element and micro lens array component, the optical element is required to have a beam angle θ of around 20° to 30°. When the distance between an optical element and a micro-lens array is too large, the beam becomes wider than the lens diameter and causes optical loss, and then, may enter the micro-lens array component in the neighboring channels as stray light to cause a serious damage to quality of signals. Therefore, the optical path length between the optical element and the lens should satisfy the expression of 2 L<250/tan (θ/2). For instance, if the approximate tangent θ is equal to 25°, the optical path length L is optimal at around 500 μm and below. As described above above, the light-emitting-element and micro lens array component should be arranged with close proximity around several hundreds μm.
b) AS described in a) above, in an optical module with closely arranged optical parts, in particular, alignment of micro-lens array and light-emitting element needs extreme accuracy. For instance, suppose that a laser with 25 degree of full angle of output light-emitting element and multi-mode fiber with the curvature radius of 50 μm are optically coupled using a convex lens with the radius 250 μm and the curvature radius of 170 μm. It is defined in the following description that the direction of the optical axis is in the Z-axis direction, and a direction perpendicular to the optical axis is X-axial and Y-axial directions. When the optical element is displaced by ±60 μm in the Z-axial direction, and by ±8 μm in the X- or Y-axial direction from the relative location for maximum coupling efficiency of optical components, the efficiency will be deteriorated by more than 1 dB. When the optical fiber is displaced by ±17 μm in the Z-axial direction, and by ±16 μm in the X- or Y-axial direction, the efficiency will be deteriorated by around 1 dB. In particular, when an optical element, a lens, and a fiber are coupled, positional displacement in the X- or Y-axial direction significantly affects the efficiency of optical coupling, and therefore, this gap should be suppressed within several μm to several tens of μm.
Furthermore, the multi-channel optical modules generally need the following requirements.
c) In order to secure reliability of optical elements, the optical elements need to be air-sealed, prevent inflow of outside air. Thus, optical elements are surrounded by inorganic materials such as metal, ceramics, and glass. However, air-sealing windows, which transmit signal light rays, are made of glass or translucent ceramics.
d) Optical modules need optical connectors to be inserted for exchanging optical signals with the outside. In the optical system comprising an outer optical connecter and an inner optical module, pins for alignment are commonly used to fix alignment. Since some stress is loaded on pins for alignment with insertion of the optical module, some sort of structure is necessary to fix the pins in the optical module. EP Patent Application No. 1310811A2 (Patent Reference #1) discloses an example of optical modules satisfying the above requirements a) to d).
FIG. 29 illustrates an aspect of a conventional technique of example 1. An optical element 112 is mounted on a plate 2904, and is easily air-sealed by fixing the plate 2904, a LIP 2903, and a lens plate 2905 with low melting point glass or solder. The distance between the lens plate 2905 and the optical element 112 is adjusted with the thickness of the LIP 2903, and the two components can be arranged at positions closed to each other. An alignment pin 101 is firmly fixed on an overlay 2901 mounted on the lens plate 2905. The alignment pin 101 is not directly fixed on the lens plate 2905, because the lens plate 2905, which is made of fragile materials such as glass and ceramics, is weak to a bending stress and broken by an intensive stress to the fixed component, when the alignment pin 101 is directly fixed to the lens plate 2905. The point stress on the alignment pin 101 can be changed to the surface stress on the overlay 2901 by fixing the alignment pin 101 on the overlay 2901. As a result, mechanical reliability is improved by mounting the overlay 2901 on the lens plate 2905.